It is well known that arcing occasionally occurs within dynamoelectric machines, such as electric generators, and in particular near the ends of the stator coil windings. Such internal arcing, which may have a variety of causes, can lead to damage to the machines, resulting in equipment down-time and costly, long-term outages.
Because arcing in generator stator coils can lead to the formation of circulating currents, overheating and further damage to the coil strands and insulation, detection of arcing within the stator windings is highly desirable. The present method of detection is the Radio Frequency (RF) monitor. This operates on the principle that any arcing in the generator will cause an increase in RF activity, which can be detected by the RF monitor. An example of such a device is disclosed in application Ser. No. 250,206, filed on Sept. 28, 1988, which application is assigned to the present assignee, and is incorporated herein by reference.
The difficulties with this method of detection are twofold. First, an increase in RF activity is not necessarily limited to that caused by arcing within the generator. Exciter and rotor grounding brush noise and AM radio transmissions, for example, can also cause such an increase. Therefore, other data must be analyzed to ascertain whether the true cause of the RF disturbance is the stator coils.
A second disadvantage is that, even if the source of the RF disturbance is determined to be within the generator, it may be difficult to obtain an accurate indication as to where inside the generator the arcing is occurring. Typically, with many prior art RF monitors, in order to determine the location of the arcing, a visual inspection and/or a transposition test must be performed. This can take up to eight hours to perform, unnecessarily increasing the duration of generator down time.
While the device disclosed in the above-referenced application can remove these uncertainties, such an RF monitor necessitates the use of sophisticated electronic circuitry to accurately analyze the RF signals. Moreover, a special mounting device is required to accurately position such a monitor, such a mounting device being disclosed in application Ser. No. 164,938, filed Mar. 7, 1988, which is also assigned to the present assignee, and incorporated herein by reference.
Along with an increase in RF activity, it is also inherent in stator coil arcing that a spark is produced. Furthermore, a spark gives off light which can be readily detected. It would be advantageous to utilize this phenomenon in the detection and location of arcing in generator stator coil ends. British Patent No. 1,476,527, published on June 16, 1977, discloses an arrangement for detecting sparking at the brushes of electrical machines. Although the device is shown to include a light guide for transmitting spark light pulses to a photo-detector, this British Patent is specifically geared to the detection of commutator, or slip ring, brush arcing and therefore utilizes electronic circuitry which is "tuned selectively" to the frequency parameters of brush sparking. This selectivity, at least in part, ensures that the influences of external light sources are excluded. Since this device utilizes this selectively filtered signal from a cable to detect arc intensity from a known source, it is not capable of detecting random sparking within a generator, as well as accurately indicating the position of arcing from a random source.